1. Field of the Invention
The present invention relates to a servo valve control method and apparatus, and, more particularly, to a servo valve control method and apparatus which employs a triplex controller as a unit for controlling a triple-coil servo valve, which is the subject to be controlled, the servo valve control method and apparatus being able to satisfactorily control the turbine in a thermal power plant, an atomic power plant or the like.
2. Description of the Prior Art
There has been known a method of controlling the triple-coil servo valve which is constituted as disclosed in "The SPEEDTRONIC MARK IV control (tm), a distributed fault tolerant gas turbine control system."; ASME 83-GT-106 in such a manner that its control system comprising coils, a servo amplifier and controllers is tripled.
The control system for the triple-coil servo valve has been arranged in such a manner that the output from each of the controllers is supplied to each of the coils via the servo amplifier so as to control the triple-coil servo valve by the total ampere's turn of the coils. Therefore, in a case where there is a fault in any one of the three systems, the residual two systems compensate the faulty system to continue the desired operation. However, if there are faults in the two systems, a decision is made that the system has been tripped.
A similar contents has been disclosed in "Turbine Digital Control and Monitoring (DCM) system"; ASME 88-JPGC/Pwr-33.
A 2-out of 3-logic system has been disclosed in "DIGITAL 2-OF-3 SELECTION AND OUTPUT CIRCUIT"; U.S. Pat. No. 4,857,762. In this literature, a method of driving the relay of a circuit, which employs an optcoupler of the 2-out of 3-logic structure, that is a digital 2-out of 3-logic structure, has been disclosed. According to the above-described disclosure, although a fault signal is selected in a case where faults have been found in the two systems, no description has not been made about an adjustment control signal.
Furthermore, as for the triple-coil servo valve, a structure, which is arranged in such a manner that its actuator is in the form of a tripled system, has been disclosed in "TRIPLE REDUNDANT ELECTROMECHANICAL LINER ACTUATOR AND METHOD"; U.S. Pat. No. 4,521,707.
In addition, a servo valve control system arranged in such a manner that its controller is tripled has been disclosed in "MULTISYSTEM CONTROL APPARATUS" (see Japanese Patent Laid-Open No. 59-85501). The above-disclosed structure is arranged to have the tripled controller so that the servo valve can be controlled if the two systems have become faulty. In this state, a standby system, which has been previously provided, is selected in place of the active system. Furthermore, a structure of the triple-coil servo valve control method has been disclosed in "Digital Electric Hydraulic Control Apparatus for Atomic Turbine", Vol. 36, No. 347, August 1985, "Thermal & Atomic Power Generation", which is arranged in such a manner that, if the two coils are disconnected, the residual coil acts to continue the operation. However, the above-described literature has not disclosed a specific means. Furthermore, no description has been made about the operation to be performed in a case where the servo amplifier or the controller has become faulty or a system constituted by combining the above-described components has become faulty.
According to any one of the above-described conventional structures, the operation of the plant has been shut down in a case where the two systems of all of the control systems have become faulty and as well as the residual system is normal or in a case where the faulty system partially contains a normal component.
Since the central control valve of a plant employs the triple-coil servo valve in order to improve reliability, the trip of the triple-coil servo valve usually causes the shutdown of the plant. However, the shutdown of a plant has been usually avoided recently in order to eliminate the social influence.
Furthermore, since the conventional method of controlling the triple-coil servo valve encounters a disturbance generated in the control system thereof if the controller has become faulty, the servo amplifier output from the control system, which includes the faulty controller, is separated from the overall system. However, in a case where the controller is normal and as well as the servo amplifier has become faulty, the servo amplifier output is not separated because the controller is normal. As a result, a problem arises in that a disturbance will be generated in the control system.